To evaluate a degraded state of a storage battery, charging/discharging a storage battery with a constant current between a full discharged state and a full charged state to obtain a QV curve has been known. Since the degraded state of the storage battery is correlated with the QV curve, the degraded state of the storage battery can be evaluated by analyzing the QV curve.
When the storage battery is evaluated by this evaluating method in an electric power storage system (ESS) in which a load fluctuates constantly, the charging/discharging of the power storage system needs to be stopped temporarily. In addition, the electric power having been charged in the storage battery needs to be dumped or stored temporarily in another storage battery.